Among antimicrobial cephalosporin derivatives, those wherein the 2-position atom of the isocephem skeleton is a sulfur atom have been described in the Japanese Unexamined Patent Publication No. 201191/1988, those wherein the 2-position atom is an oxygen atom have been described in the Japanese Unexamined Patent Publication No. 152383/1987, and those wherein the 1-position atom is an oxygen atom have been described in the Japanese Unexamined Patent Publication No. 313788/1988 etc.
However, the cephalosporin derivatives in the present invention are structurally different from the known cephalosporin derivatives.